The established existing art for the optical measurement of feature widths on masks of the semiconductor industry is the use of microscopes in the visual (VIS), ultraviolet (UV), and deep ultraviolet (DUV) spectral regions using dry objectives. Dry objectives are characterized in that air is present between the specimen and the first optical surface of the microscope objective immediately adjacent to the specimen. As a consequence of this, the theoretical maximum possible numerical aperture NA is equal to 1; in practice, values of at most NA≈0.95 are attainable, and have in fact been achieved. The scope for higher apertures, i.e. higher resolution, has thus been exhausted.